Boost surging at and near WOT

[willisdaye]

I've read a lot of posts about this, and tried the easy stuff first (clean MAF, check for vacuum leaks, new AF, reset ECU) but wanted some opinions. It's an 08 OBXT I recently picked up. It has a newly built EJ25 and is stock except for a Megan Racing turboback exhaust. PO had it tuned at a reputable local shop. I'm in the market for a different exhaust because I'm not a huge fan of how the fart cannon looks, and would like something a bit better quality - but that's a story for another day. I'll also definitely get in touch with the tuner about all of this but wanted to share log results here.

It drives very well at less than 60% or so throttle, has lots of torque. It acts differently under heavy boost though. It will build a good amount of boost initially but will hold back eventually, and will start surging right at the spot where it should be really pushing me back in the seat. Sometimes the bucking as a result of this is decently violent. I got around to getting a datalogger set up, so I caught it on a log today, just a quick WOT pull in 3rd. Boost seems like it's having trouble settling, between 10 and 15 psi (relative values). This leads me to think it's BCS / wastegate related, but I wanted to see what others think of the datalog before I start pulling things off and testing them (and talking to the tuner). That graph below is RPM and relative manifold pressure over time. There were a couple knock events but they happened at the very beginning and end of the pull.

 

 

romraiderlog_20221022_211259.csv

Edited October 23, 2022 by willisdaye

[Haze]

Your best bet is probably getting in touch with the tuner. They may have seen this before. I have a bit of experience with Romraider but it could take me hours of guessing, datalogging and internet searching to figure out what might be going on here.

I would start by datalogging "boost error", "wastegate duty cycle", "turbo dynamics integral" and "turbo dynamics proportional". The turbo dynamics tables can be set to make huge changes to wastegate duty cycle if boost error gets too high. I suspect this is a mechanical issue that the ECU is trying to compensate for but can't do so in a smooth manner.

[m sprank]

Your load values are bouncing, your timing values are bouncing, your boost is bouncing as is your wastegate and mass a/f.  Its all related.  Have a learning view?  I suspect a leak or wastegate issue.  I doubt a reputable tuner would have left the car this way.

[silverton]

MAF.

[m sprank]

Could be as simple as a good cleaning/connection issue/failing sensor.  Start with a visual inspection and cleaning of the senor and the terminals. 

[willisdaye]

Thanks for the replies. I haven't got a chance to take more logs but I agree that the MAF is suspect. I've cleaned it a couple times with a can of the good stuff but it could be time for a new one. I'll talk to the tuner tomorrow and see if he has any guidance. In the meantime, here's a picture of the MAF. Isn't this engine supposed to have Denso 197-6040? Also I hope that number at the bottom isn't a date code for 7-22-2014...

[silverton]

That is likely the correct sensor, and the screws look untouched, I've found cleaning will only fix like 1/5 cars. 

[m sprank]

Sensor appears OEM and original. 

[Infosecdad]

10 hours ago, willisdaye said:

Thanks for the replies. I haven't got a chance to take more logs but I agree that the MAF is suspect. I've cleaned it a couple times with a can of the good stuff but it could be time for a new one. I'll talk to the tuner tomorrow and see if he has any guidance. In the meantime, here's a picture of the MAF. Isn't this engine supposed to have Denso 197-6040? Also I hope that number at the bottom isn't a date code for 7-22-2014...

The part number is the first one.

That's the OEM (and probably original) MAF for your car.

https://parts.subaru.com/p/49232594/22680AA31A.html

 

[Colico]

I'm very new to to the boosted subi field. But have decent experience with VAG ( Volkswagen Audi group). In that case, I probably would say the MAF, but your dasta logging sheets look like many Subaru sheets I've seen with Stock BSA not handling everything effective, again I'm new and admit I may be totally wrong.

[willisdaye]

3 hours ago, Colico said:

I'm very new to to the boosted subi field. But have decent experience with VAG ( Volkswagen Audi group). In that case, I probably would say the MAF, but your dasta logging sheets look like many Subaru sheets I've seen with Stock BSA not handling everything effective, again I'm new and admit I may be totally wrong.

I replaced the MAF and saw a little improvement but the surging issue is still there for the most part. What do you mean by BSA? Is that like the VW name for boost control solenoid? Also, I couldn't find the tuner because it was tuned by the PO before they sold it to the shop, and didn't get the info.

Edited November 5, 2022 by willisdaye

[m sprank]

The plot thickens. 

 

There was not a lot of swing in the maf rdgs. What was there coincided with other swings. 

 

Prior to sinking the tuner, I would at a minimum pull the down pipe and inspect the wastegate and housing. Cracked housing, siezed/obstructed wastegate, etc. If the wategate arm is adjustable anyone with a crescent wrench can f up the boost control. Forum troubleshooting can be a shot in the dark based on past experience.  Vacuum leaks, poper pill size, failing boost control solonoid (rare, but it happens), wastegate operation, fuel, air, timing. Heck is it a catted uppipe and the cat broke free? What does it take to make boost?  Like climbing a ladder step on every rung. Skipping rungs leads to slips and falls. 

 

Easy answer is drop  at off at my house.  I will have it figured out and fixed before the end of the day, lol. 

[Max Capacity]

I'm just saying, that yellow dot on that MAF. Makes me think it's from a junkyard. Car was build May 28 2004.

I just walked out and checked the MAF on the 09 Spec B, no paint marks either.

 

Here's the OEM from my 05 wagon. Notice, no paint marks.   But, I see you've replaced it already...

 

[m sprank]

I have seen "the paint" more times than I can count on OEM sensors.  It is normal for assembly line techs to mark torque and connections with paint.  Even at my own shop we marked the same way.  You know you torqued it or checked the connection, because you marked it.  Subie likes yellow and green. 

Recycle yards (Junk yards) mark differently.  Those marks are far more obvious that they denote used.  They trend to use pink markers.  LKQ my arse.  Never accept LKQ, always demand OEM for repairs.  I feel another rant starting. 

[Max Capacity]

2 hours ago, m sprank said:

I have seen "the paint" more times than I can count on OEM sensors.  It is normal for assembly line techs to mark torque and connections with paint.  Even at my own shop we marked the same way.  You know you torqued it or checked the connection, because you marked it.  Subie likes yellow and green. 

Recycle yards (Junk yards) mark differently.  Those marks are far more obvious that they denote used.  They trend to use pink markers.  LKQ my arse.  Never accept LKQ, always demand OEM for repairs.  I feel another rant starting. 

No rant from me... I was just showing mine...no,you don't need to show me your's, LOL.

[willisdaye]

Wish I could have one of you guys look at it! Thanks for all the input. There's a self service garage with a lift nearby. I'll try to take it there this weekend and see if I can find anything with the downpipe off. Everything down there is basically rust free so it should go smoothly...fingers crossed.

Also gonna look at my driveshaft, I'm hearing / feeling some clunking during quick take offs and poorly made upshifts...

[Pleides]

Our center support area has a little more play in it than most I've seen in other AWD driveshafts. These car's 5-speeds are kinda clunky no matter how much you modify the car to make it not so. I have a Group N trans+engine mounts, urethane diff and subframe bushings, and urethane shift bushings and I still upset the transmission or get noises when I think I've changed gear perfectly. Rest your elbow on the center console while driving and see if anything odd is picked up. Chances are, your driveshaft is fine.

I'd recommend grabbing a set of gaskets for whatever you plan to remove to inspect the car, just FYI.

Edited November 9, 2022 by Pleides

[m sprank]

Driveshaft is non-serviceable as well.  Rare to have an issue unless making bigger power.  Gaskets are a must.  PB Blaster as well.  Prepare for seized turbo nuts/studs.  You would be amazed at what heat cycling does to torque.  Broken bolts, wrenches, knuckles.  Its what makes a Subaru a Subaru, LOL. 

[willisdaye]

39 minutes ago, Pleides said:

Our center support area has a little more play in it than most I've seen in other AWD driveshafts. These car's 5-speeds are kinda clunky no matter how much you modify the car to make it not so. I have a Group N trans+engine mounts, urethane diff and subframe bushings, and urethane shift bushings and I still upset the transmission or get noises when I think I've changed gear perfectly. Rest your elbow on the center console while driving and see if anything odd is picked up. Chances are, your driveshaft is fine.

I'd recommend grabbing a set of gaskets for whatever you plan to remove to inspect the car, just FYI.

Good to know. Its a repeating noise that reminds me of when my CV joints went bad on my 97, just lower pitched and coming from under the car. Because of that I'm wary of the U-joints. There's 175k on the drivetrain after all. If I need a new shaft I'm not sure if I'd go used OEM or get one with serviceable joints. A shop guy was telling me he just gets single piece ones made but I don't know where I'd get that done where I live.

The exhaust is aftermarket and I actually don't know what model it is or even if it was originally made for the car. The turbo flange should be the same since it's the factory turbo so that gasket should fit if I can find one. I'll have to see about the other side, might have to make a gasket for it if the existing one is in rough shape.

Also just now realizing that everything's gonna be real hot when I get to the shop. I wonder how that will affect the difficulty of removing the hardware (Definitely won't be easy on my hands at least!) I guess it depends on if the bolts+studs are different materials. The shop charges per hour for lift usage so I'm not sure how long I want to wait for it to cool 

Edited November 9, 2022 by willisdaye

[Pleides]

Bring some heat-insulated gloves!

The company below sells serviceable driveshafts if you need one. The U-Joint is the most common point of failure on these. Many report rather sudden failures of theirs (which can cause further damage or safety concerns) so if you find yours to be going then I'd recommend one ASAP. You may also have a bad wheel bearing if the noise sounds rotational. ALl four of mine on the wagon have been replaced at least once.

https://driveshaftstore.com/Subaru-Legacy-Driveshaft-2005

[m sprank]

Park it outside overnight and push it in the morning when they open, lol.  Its going to be much hotter than you want to work on. 

[willisdaye]

Big updates:

Both the surging and the clunking noise appear to be caused by the poorly fitting and poorly installed aftermarket exhaust system.

There's a missing nut on the downpipe/turbo flange, and since it's the bottom one (furthest away from the others) it's likely compromising the seal and could definitely be the cause of the surging. 

The clunking is caused by the oxygen sensor colliding with the undercarriage heat shields when the car shudders during clutch actuation. Since the system isn't made for the outback (or any Subaru for all I know), the bung isnt at the proper angle.

It seems like it's a good time to get a whole new exhaust system on it at this point. I might be able to get a deal on Invidia components through a local shop. If so, I'm thinking I'm going to install Invidia Uppipe, Catted Divorced wastegate downpipe, and the Q300 quad tip catback they make for the LGT. I read a few forum posts here and on the outback forum that it should fit without modifying the bumper if I can bend the hangar mounts or just install adjustable height hangars so the tips sit below the bumper. I think that's my course of action for now, will update once I get confirmation from the shop. Of course I'll have to have it tuned after all this... I'd imagine the 10 miles to the tuner wouldn't be too hard on the engine if I baby it, the new system should be only a little more restrictive than the existing one.

[Pleides]

Nameless makes a downturn exhaust for your car, if that's your thing. If you don't plan on changing the bumper then it's a good choice.

[willisdaye]

Got the new exhaust parts on. Fits pretty nice! The Y pipe hangs a little low for my liking but that's alright. The sound is really nice, not nearly as loud and ratty as the old one but now it has a refined growl. I spoke to a tuner and found that the boost issue seems to be due to the poor tune. Whoever open source tuned it before me didn't really know what they were doing and raised the boost limits without changing the boost targets any higher than stock. Without cats in the downpipe but mostly stock on the maps, the ecu holds back when it gets boost faster than expected. Seems I'll be looking good once I have it tuned.  I'm also going to upgrade to a DW65C fuel pump beforehand, given the mileage on the car and to make the most out of the new system/tune.

[KZJonny]

Fwiw - I was told by the tuner I’m working with, (Dave at Cryotune, so someone who sees a *few* Subarus) to get the AEM340 fuel filter and avoid the DW unit.

Didn’t ask why exactly, he just said they had better results with them, and would prefer to tune with it. This is a sentiment I’ve seen a couple times on here.

Ymmv.